1. Field of the Invention
The invention relates generally to percussion, or hammer, bits. More specifically, the invention relates to a secondary retainer for a downhole hammer bit.
2. Background Art
Percussion bit systems are often used in drilling or boring through the earth's surface. In a percussion bit system, a percussion hammer is used to drive the percussion bit into the ground using the reciprocating action of a piston to energize the bit.
FIG. 1 illustrates a conventional percussion bit assembly design 100. Percussion hammer assembly 100 comprises a hammer case 101 that connects to a lower end of a drill string (not shown) through a threaded pin connection 145. The lower end of the hammer case 101 is threadedly engaged with driver sub 102. A plurality of splines (not shown) disposed on the driver sub 102, engage a plurality of splines 115 disposed on a shank 112 of a hammer bit 110, and rotatatively drive the bit 110. A retainer, conventionally a split ring, 113 is disposed around an upper end of the shank 112 of the hammer bit 110 and abuts the driver sub 102. The split ring 113 retains the hammer bit 110 in the hammer assembly 100. The split ring 113 may be held in place, initially, by an elastic ring, or o-ring, 116 to facilitate assembly of the bit and driver sub with the hammer case. The split ring is confined by the inner wall of the hammer case 101 to maintain ring-to-bit engagement. The upper end of the hammer bit 110 includes a piston strike surface 148 and a foot valve, or blow tube, 147. The lower end of the hammer bit includes a head 111.
The hammer assembly includes a control tube 143 and an annular piston chamber 146. Pressurized air moves a piston 142 in a reciprocating motion inside the annular piston chamber 146. A check valve 144 is used to communicate one-way flow between the control tube 143 and the drill string (not shown). The lower end of the piston 142 is adapted to strike the piston strike surface 148, thereby imparting kinetic energy to the bit 110.
During certain operations performed with a hammer tool, it may be necessary to reverse drill pipe rotation, thereby causing the driver sub 102 to back off, or unthread, from the hammer case 101. Occasionally, a driver sub will unintentionally back off downhole due to torsional oscillations, known as “stick-slip”, of the drill string. If the driver sub backs off, the bit 110 and the driver sub 102 remain at the bottom of the borehole. When this occurs, the elastic ring 116 that restricts the movement of the split ring 113 during installation often fails and permits the split ring to fall away out of engagement with the bit. Consequently, the split ring does not fulfill the function of retaining the bit in the driver sub assembly. Therefore, when the driller attempts to screw back into the driver sub with the hammer case, the bit is left in the hole when the hammer is extracted. Sometimes the split ring segments lodge in between the driver sub 102 and the borehole wall, preventing the hammer case 101 from screwing back over the driver sub 102. The bit left in the hole has to be retrieved later through a costly fishing operation.
A segmented retainer sleeve 24, as shown in FIGS. 2 and 3 can be used to retain the bit 210 to the driver sub assembly after a driver sub backs off. FIGS. 2 and 3 show a conventional hammer assembly 200 comprising a driver sub 202, a segmented retainer sleeve 24, a hammer case 201, and a bit 210. U.S. Pat. No. 5,803,192 shows an example of such a conventional bit retainer.
The segmented retainer sleeve 24, as shown in FIGS. 2 and 3, includes a ring portion 24A, which seats about a reduced diameter segment 25 of the bit 210. Ring portion 24A, in combination with shoulder 24B, limits reciprocal travel of the drill bit 210. Skirt extension 26 has a lesser wall thickness than the ring portion 24A and slidably engages the bit 210. Skirt extension 26 nests within a counterbore 27 shown formed in the upper end of the driver sub 202. During a drilling operation, should the driver sub 202 back off, the segmented retainer sleeve 24 should remain in place about the bit because the skirt extension 26 is radially confined. Joining the hammer case 201 with the driver sub 202 after the driver sub 202 backs off may be accomplished by rotating the hammer case 201 on the driver sub 202. A drawback of this design is that, as a result of the counterbore 27 in the driver sub 202, the spline contact area between the splines 33 on the driver sub and the splines (not shown) on the bit 210 is reduced. A reduction in the spline contact area may result in rapid wear and breakage of the splines.
Accordingly, there exists a need for a downhole hammer bit retainer that retains the bit within the driver sub in the event the driver sub backs off downhole and that maintains the mechanical integrity of the driver sub and drill bit.